Conventional varifocal objectives comprise several (usually four) components of which two, generally the second and the third one, are axially shiftable to vary the overall focal length. The first of these axially shiftable components, termed a variator, determines that overall focal length whereas the second one, termed a compensator, maintains the position of the image plane constant throughout the zoom range. Thus, the positions of these shiftable and usually negatively refracting components are correlated in a predetermined manner according to a nonlinear law of relative displacement. Frequently, the objective also has a (usually positively refracting) front component which is limitedly axially adjustable, or includes an axially adjustable lens member, for focusing purposes. Alternatively, focusing (i.e. a change in the object distance) can also be carried out by modifying the relative position of the axially shiftable components used for zooming, yet this is more complicated since the extent of the supplemental shift required for focusing varies with the selected overall focal length.
In commonly owned U.S. Pat. No. 4,008,951 there has been disclosed a control mechanism in which the position of the second shiftable component, i.e., the compensator, is changed in response to stepping pulses generated upon the displacement of the first shiftable component, i.e., the variator, to maintain the requisite positional correlation of the two components.
A somewhat similar system has been described in German published specification No. 2,410,744 and corresponding U.S. Pat. No. 4,043,642 wherein the variator and the compensator are provided with respective position sensors working into a computer which sends corrective signals to the compensator drive for establishing the proper correlation. The operation of the computer may be modified to focus the objective upon an object located at a certain distance therefrom. The intervention of a computer in the positioning of the compensator introduces a certain delay into the adjustment of the compensator, thereby limiting the speed of the zooming operation. This delay, and the complexity of the computer, is aggravated if focusing is to be performed by modifying the positional correlation of the two shiftable components.